Exposure apparatus for color cathode ray tubes
BACKGROUND OF THE INVENTION 
     This invention relates to an exposure apparatus for production of a phosphor screen of a cathode ray tube and more particularly, relates to an exposure apparatus for production of the phosphor screen of multineck color cathode ray tubes. 
     Recently, for the color display tube with a large-sized screen and a sufficient high degrees of brightness and resolution, a multineck color cathode ray tube has been proposed. For example U.S. Pat. No. 4,714,856 discloses a cathode ray tube, which includes a phosphor screen formed on an inner surface of a single glass panel and 12 electron gun assemblies provided in necks. In accordance with the electron gun assemblies, the phosphor screen has 12 effective areas for reproduction of images. Each electron gun assembly emits three electron beams for bombarding phosphor stripes of red, green and blue formed in the corresponding effective area of the phosphor screen. Deflection yokes are provided outside funnels which connect the necks to the panel, respectively. 
     During operation of the tube, the electron beams emitted from the electron gun assemblies scan the corresponding effective areas due to the deflection yokes, respectively, and thus images with the high brightness and resolution can be reproduced on the large phosphor screen, entirely. 
     For formation of such phosphor screen with large size, an exposure apparatus shown in FIG. 1 can be considered. Namely, as an extention of the exposure apparatus used for formation of the phosphor screen with an ordinary size, such as 20 inches-size or the less, the exposure apparatus 1 includes a housing 2 divided into cells 2a, 2b, 2c and 2d, a plurality of light sources 3a, 3b, 3c and 3d, such as a mercury lamp or xenon lamp, for exposing predetermined regions corresponding to the effective areas of the phosphor screen, respectively. Correction lenses 4a, 4b, 4c and 4d and intensity correction filters 5a, 5b, 5c and 5d are placed above the light sources 3a, 3b, 3c and 3d for correct exposure. On the top of the apparatus 1, is placed a panel 6 with a photosensitive layer 7 formed on an inner surface of the panel 6 for forming phosphor stripes of red, green and blue, and light absorbing stripes between the phosphor stripes. A shadow mask 8 is placed inside the panel 6. 
     Many problems have occurred when the apparatus shown in FIG. 1 is used for formation of the large-sized phosphor screen for the multineck tubes. Namely, it is hard to uniformely form the phosphor stripes and light absorbing stripes in sizes and pitches, since keeping the intensity of all light sources to be constant for a long duration is not easy. Also, since the distance between neighboring light sources are very small, mechanism of cooling system and moving system of the light sources are extremely complicated and required to be accurate. 
     In Japanese Utility Model Disclosure No. 51-30462, is disclosed an exposure apparatus for formation of the light absorbing stripes coating of the phosphor screen. The apparatus includes a light source, such as a mercury lamp, and three light conductors, each of which is composed of a plurality of optical fibers, for transmitting the lights from the light source. Inlets of the conductors are bundled and placed adjacent to the light source so as to receive the lights from the light source. Outlets of the conductors are disposed at predetermined positions, respectively. 
     By using the apparatus, since the light absorbing stripes can be formed by only once exposure, the duration of exposure time can be reduced. However, since intensity of the light emitted from the lamp significantly decreases at the point slightly shifted from a center of the lamp where light intensity is maximum value, illuminance amounts emitted from the outlets are different. As the result, the stripes are not formed, uniformly. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an exposure apparatus which can uniformly form a phosphor screen for large-sized color cathode ray tubes. 
     Another object of the invention is to provide an exposure apparatus with a plurality of secondary light sources which can uniformly emit exposure light. 
     Further object of the invention is to provide an exposure apparatus with a simple construction. 
     Still further object of the invention is to provide an exposure apparatus suitable for multineck color cathode ray tubes. 
     Therefore, the invention may provide an exposure apparatus for color cathode ray tubes having a single faceplate with a phosphor screen on an inner surface thereof comprising light source means including an initial light source for emitting ultraviolet light, exposure means including a base for supporting the faceplate with a photosensitive layer formed on the inner surface thereof to form the phosphor screen and having a plurality of sub-regions, a plurality of secondary light sources emitting exposure light for exposing respective sub-regions of the photosensitive layer and moving means for shifting each position of the secondary light sources at predetermined positions, and light distribution means for equally distributing a part of ultraviolet light emitted from the light source means to the secondary light sources. 
     According to the invention, since ultraviolet light with equal intensity are transmitted to the secondary light sources due to the light distribution means, all of exposure light emitted from the secondary light sources have almost same intensity. Consequently, the phosphor screen with large size is uniformely formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of a conventional exposure apparatus. 
     FIG. 2 shows a cross-sectional view of a color cathode ray tube with a large-sized phosphor screen formed by using an exposure apparatus according to the invention. 
     FIG. 3 shows a perspective view of a shadow mask shown in FIG. 2. 
     FIG. 4 shows a cross-sectional view of an exposure apparatus according to the preferred embodiment of this invention. 
     FIG. 5 shows a cross-sectional view of the panel for explanation of the operation of the exposure apparatus shown in FIG. 4. 
     FIG. 6 shows a side view of a light distribution member shown in FIG. 4. 
     FIG. 7 shows a cross-sectional view of a secondary light source shown in FIG. 4. 
     FIG. 8 shows a cross-sectional view of an exposure apparatus according to another embodiment of the invention. 
     FIG. 9 shows a plane view of one end of light conductors shown in FIG. 8. 
     FIGS. 10A, 10B and 10C show a plane view of another end of the light conductors shown in FIG. 8. 
     FIG. 11 shows a plane view of an inlet of a bundled light conductors. 
     FIG. 12 shows a side view of a part of the exposure apparatus according to the other embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Preferred embodiment of this invention will be explained with reference to the accompanying drawings. FIG. 2 shows a color cathode ray tube with a large-sized phosphor screen which has been formed be using an exposure apparatus according to the preferred embodiment. In FIG. 2, the cathode ray tube 20 includes a single panel 21 with a phosphor screen 22 formed on an inner surface of the panel 21. The phosphor screen 22, which comprises a plurality of phosphor stripes horizontally extending for emitting red, green and blue lights and light absorbing stripes interposed between the phosphor stripes, is divided into sixteen scanning sub-regions 22a, 22b, 22c and 22d, four scanning regions vertically and four scanning sub-regions horizontally. Corresponding to the sub-regions 22a, 22b, 22c and 22d, sixteen electron guns 23a, 23b, 23c and 23d are provided in sixteen necks 24a, 24b 24c and 24d, respectively. Each electron guns 23a, 23b, 23c and 23d emits three electron beams 25a, 25b, 25c and 25d for bomberding the phosphor stripes in corresponding scanning sub-regions 22a, 22b, 22c and 22d for emitting red, green and blue lights. In the drawing, three electron beams are shown by a single line for simplification. The necks 24a, 24b, 24c and 24d are connected to the panel 21 by a funnel 26. Sixteen deflection yokes 27a, 27b, 27c and 27d are attached to the outside of the funnel 26 for deflecting the electron beams 25a, 25b, 25c and 25d on the scanning sub-regions 22a, 22b, 22c and 22d, respectively. 
     In the panel 21, a shadow mask 28 is disposed by supporting a plurality of panel pins (not shown) through a mask frame 29 so as to have a predetermined distance between the phosphor screen 22 and the shadow mask 28. As shown in FIG. 3, the shadow mask 28 has sixteen effective areas 28a, 28b, 28c . . . and 28 p with a plurality of apertures for passage of the electron beams 25a, 25b, 25c and 25d, and non-effective portion 30 without aperture for isolating the effective areas 28a, 28b, 28c . . . and 28 p. The effective areas 28a, 28b, 28c . . . and 28p respectively correspond to the scanning sub-regions 22a, 22b, 22c and 22d. The non-effective portion 30 prevents the electron beams from bomberding neighboring scanning sub-regions. 
     During operation of the tube 20, the electron beams 25a, 25b, 25c and 25d emitted from the electron guns 23a, 23b, 23c and 23d are deflected around deflection centers 31a, 31b, 31c and 31d so as to scan over the scanning sub-regions 22a, 22b, 22c and 22d, respectively. Consequently, large-sized images are reproduced on the phosphor screen 22 as a composite image of unit images reproduced on the scanning sub-regions 22a, 22b, 22c and 22d by deflection of the beams 25a, 25b, 25c and 25d. 
     The exposure apparatus 40 used for formation of the phosphor screen mentioned above is shown in FIG. 4. The exposure apparatus 40 according to the invention has an exposure device 41, a light source device 42 for emitting ultraviolet light and a light distribution device 43 for equally distributing ultraviolet light from the light source device 42 to the exposure device 41. 
     The exposure device 41 has a rectangular box-shaped frame 44 with the upper side opening. The space inside the frame 44 is divided into sixteen compartments 45a, 45b, 45c and 45d by dividing plate 46 provided in a lattice form. The compartments 45a, 45b, 45c and 45d correspond to the scanning sub-regions 22a, 22b, 22c and 22d of the phosphor screen to be formed. In the compartments 45a, 45b, 45c and 45d, secondary light sources 47a, 47b, 47c and 47d, correction lenses 48a, 48b, 48c and 48d, and light quantity correction filter 49a, 49b, 49c and 49d are respectively disposed to construct exposure unit. The secondary light sources 47a, 47b, 47c and 47d are supported by shift tables 50a, 50b, 50c and 50d in order to be shifted the positions thereof in a horizontal direction (left and right directions in FIG. 4). 
     On the top of the frame 44, a supporting plate 51, which has sixteen openings corresponding to the compartments 45a, 45b, 45c and 45d, is provided for supporting the panel 21. On an inner surface of the panel 21, a photosensitive layer 52 is formed for formation of the phosphor screen. The photosensitive layer 52 has sixteen exposing sub-regions 52a, 52b, 52c and 52d to be exposed by ultraviolet light emitted from the secondary light sources 47a, 47b, 47c and 47d. The shadow mask 28 with the effective areas 28a, 28b, 28c and 28d and non-effective portion 30 is disposed in the panel 21 by support of the pins through the mask frame 29, as previously described. 
     Shielding plates 53 are provided on the supporting plate 51 in a lattice form same as the dividing plate 46 to divide the space inside the panel 21 into sixteen areas corresponding to the exposing sub-regions 52a, 52b, 52c and 52d. The shield plates 53 eliminate an undesirable interaction between ultraviolet light radiated from neighboring secondary light sources during exposure. Namely, in the type of the color cathode ray tube shown in FIG. 2, the shadow mask 28 has the non-effective portion 30 for blocking passage of the undesirable electron beams in order that images (rasters) reproduced by the electron beams do not overlap with each other, even if the electron beams are scanned beyond the predetermined scanning sub-regions. 
     Also, during exposure of the photosensitive layer 52, the non-effective portion 31 blocks exposure light from the secondary sources 47a, 47b, 47c and 47d to expose neighboring exposing sub-regions 52a, 52b, 52c and 52d in some degree. However, such undesirable exposure light cannot be completely blocked by the non-effective portion 31 alone. Therefore, the shield plate 53 is necessary to successfully expose the photosensitive layer 52. Namely, as shown in FIG. 5, the shield plate 53 can completely shield undesirable exposure light shown by the chain lines. Accordingly, due to the shield plate 53 and non-effective area 31, the exposure light is prevented from mutual interference. Thus, the exposing sub-regions corresponding to the scanning sub-regions are successfully exposed by the exposure light from the secondary light source. 
     In this embodiment, all shield plates 53 extend to the extent that the distances between the surface of the shadow mask 28 and the top of the shield plates are 5 mm. So, hight of the shield plates 53 are different each other due to curvature of the shadow mask surface. 
     The light source device 42 includes a light shield case 54, an initial light source 55, such as an air-cooled ultra high pressure mercury lamp with an output power of 1 kw, which is disposed in the case 54 for emitting ultraviOlet light with wave length of 365 nm, and focusing lens system 56 for focusing the light from the initial light source 55. 
     As shown in FIG. 6, the light distribution device 43 includes a light diffusing rod 57 made of a transparent glass, such as quartz, and a plurality of light conductors 58, such as a optical fiber. On the incidence end 57a of the rod 57, a spot of ultraviolet light passing through the focusing lens system 56. An output end 57b of the rod 57 is optically contacted with one end 59 of the light conductors 58 for transferring the ultraviolet light transmitting in the rod 57. Another ends of the conductors 58 are divided into a plurality of branches 58a, 58b, . . . 58h and 58i for equally distributing a part of ultraviolet light from the output end 57b. The number of the branches 58a, 58b, . . . 58h and 58i corresponds to the number of the exposing sub-regions. Namely, the light conductors 58 are divided into sixteen branches in this embodiment. For example, the conductors 58 comprises 1600 optical fibers which are bundled at one end to have a diameter of 20 mm for contacting with the output end 57b and are divided into 16 branches, in the case of the embodiment. 
     In the light distribution device 43, ultraviolet light projected on the incidence end 57a uniformely spreads in the diffusion rod 57 during transmitting the diffusion rod 57 due to total deflection at the pheriphery of the rod 57. As a result, the ultraviolet light emitted from the output end 57b has uniform intensity. Therefore, the ultraviolet light with unform intensity is distributed to each branches 58a, 58b, . . . 58i. The ends of the branches 58a, 58b, . . . 58i are supported in the secondary light sources 47a, 47b, 47c and 47d. 
     The secondary light source 47a includes a holder 47a1, for supporting another end of the branch 47a, as shown in FIG. 7. The holder is fixed to a shift table 50a. The secondary light source 47a also includes a case 47a2, focusing lens system 47a3 for adjusting a spread angle of exposure light from another end of the branches 58a and a hole plate 47a4 provided a pin-hole 47a5 with a diameter of 1 mm. 
     The secondary light source 47a emits exposure light with a spread angle of 90°, and thus, the light can sufficiently expose a circular area with a diameter of about 40 mm on the photosensitive layer 52. It should be noted that the position of the secondary light source 47a is adjusted when exposed in order that exposure light is emitted from the position corresponding to the deflection center of the scanning sub-region (or exposing sub-regions). In other words, the pin-hole 47a5 should be positioned at the position corresponding to the deflection center. 
     During exposure, ultraviolet light emitted from the initial light source 55 is projected on the incidence end 57a. The light is transmits in the diffusion rod 57 and spreads over the diffusion rod 57. At the output end 57b, the light is uniformly divided and transmits in the branches 58a, 58b, . . . and 58i. The light is radiated from the secondary light sources 47a, 47b, 47c and 47d for exposing the exposing sub-regions 52a, 52b, 52c and 52d. Since ultraviolet light emitted from the initial light source 55 with a acute intensity distribution is distributed to the secondary light sources 47a, 47b, 47c and 47d after transmitting the diffusion rod 57 and the light conductors 58, intensity of the light radiated from the secondary light sources are uniform. Consequently, the phosphor screen can be formed, uniformly. 
     Another embodiment will be explained with reference to FIGS. 8, 9 and 10. In FIG. 8, same portions as the portions shown in FIG. 4 are given the same number in FIG. 4. 
     An exposure apparatus 100 shown in FIG. 8 has an exposure device 41, a light source device 110 and a light distribution device 120 for equally distributing ultraviolet light from the light source device 110 to the exposure device 41. 
     The exposure device 41 has the rectangular box-shaped frame 44 with the upper side opening. The space inside the frame 44 is divided into sixteen compartments 45a, 45b, 45c and 45d by dividing plate 46. The compartments 45a, 45b, 45c and 45d correspond to the exposing sub-regions 52a, 52b, 52c and 52d of the photosensitive layer 52. In the compartments 45a, 45b, 45c and 45d, secondary light sources 47a, 47b, 47c and 47d, correction lenses 48a, 48b, 48c and 48d, and light quantity correction filters 49a, 49b, 49c and 49d are respectively disposed to construct exposure unit. The secondary light sources 47a, 47b, 47c and 47d are supported by shift tables 50a, 50b, 50c and 50d. 
     On the top of the frame 44, the supporting plate 51 is provided for mounting the panel 21. The panel 21 has the photosensitive layer 52 on the inner surface. In the panel 21, the shadow mask 28 is disposed by support of the pins through the mask frame 29. The photosensitive layer 52 has sixteen sub-regions 52a, 52b, 52c and 52d to be exposed and the shadow mask 28 has sixteen effective areas 28a, 28b, 28c and 28d with apertures. 
     Shielding plates 53 are provided on the supporting plate 51 for elimination of an undesirable interaction between exposure lights during exposure. 
     The light source device 110 includes a light shield case 111, an initial light source 112, which is disposed in the case 111 for emitting ultraviolet light, and focusing lens system 113 for focusing the light from the initial light source 112. As the light source 112, an air-cooled ultra high pressure mercury lamp with an output power of 1 kw, which emits ultraviolet light with wave length of 365 nm, may be used, for example. 
     The light distribution device 120 is composed of a plurality of light conductors 121, such as an optical fiber. The conductors 121 are bundled at one end for forming an inlet 122 on which a spot of ultraviolet light through the focusing lens system 113 is projected. Another end of the conductors 121 are divided into sixteen groups so as to construct branches 123a, 123b, . . . and 123k. Each end of the branches 123a, 123b, . . . and 123k is supported by the holder of the corresponding secondary light sources 47a, 47b, 47c and 47d. 
     Since ultraviolet light emitted from the end of the branches 123a, 123b, . . . and 123k, as the exposure light, simultaneously expose the exposing sub-regions 52a, 52b, 52c and 52d for formation of the phosphor screen, it is necessary that all of the exposure lights exposing have substantially same intensity for formation of uniform phosphor screen. However, as compared with the light distribution device according to the embodiment previously mentioned, since the light distribution device 120 does not have diffusion rod, the intensity of ultraviolet light projected to each light conductors 121 differ greatly between the conductors at center and periphery of the bundled end of the conductors 121 due to an acute intensity distribution of the initial light source 112. So, if another end of the conductors 121 are divided into groups to form branches without any consideration for intensity equalization of the light emitted from the ends of the branches, it would be hard that the exposing sub-regions are exposed, equally. 
     Therefore, another ends of the conductor 121 are divided into groups at random to form branches 123a, 123b, and 123k. Consequently, the intensities of the exposure light emitted from the branches are made equal. Namely,. twenty-one light conductors 121 with same mark a, b and c in FIG. 9 are gathered each other so that the light conductors 121 are divided into three groups at random, as shown in FIGS. 10A, 10B and 10C. For example, 1600 optical fibers are bundled at one end and are divided into 16 branches at random. However, it is possible that the intensity of exposure light projected on the inlet 122 of the conductors 121 is not uniform due to influence of the focusing lens system 113. In this case, each of 16 branches is composed of another ends of the optical fibers which are selected from the fibers located in the area A, B and C of the inlet 122 shown in FIG. 11. More precisely, the optical fibers of which one ends are located in the area A are divided into 16 groups X at random. Also, the optical fibers of which one ends are located in the areas B and C are divided into 16 groups y and Z. respectively. Finally, the 16 branches are formed by gathering the groups X, Y and Z. Consequently, the intensities of the exposure light emitted from the branches are made equal. The sizes of the areas A, B and C depend on intensity distribution of the exposure light projected on the inlet 122. 
     As shown in FIG. 12, more than 2 sets of light source units 200, each of which includes a light source device 201, a light spliter 202 and light conductors 203, may be used, when the intensity of exposure light exposed on the photosensitive layer is insufficient. One ends of the conductors 203 are bundled and optically coupled with the light spliter 202. Another ends of the conductors 203 are divided into predetermined number of branches 204a, 204b and 204c. The branches 204a, 204b and 204c are unitized at ends 205a, 205b and 205c. 
     In the exposure apparatus of this invention, half-mirrors which have different reflectivity and prisms may be used for spliting ultraviolet light emitted from the light source device. 
     Also, a concave mirror which have focusing action, instead of the focusing lens in the light source device. 
     Incidentally, in the embodiment, light shielding plates 53 are provided on the supporting plate 51. However, other light shielding devices can be provided in the vicinity of the correction lenses, the light quantity correction filters or the secondary light sources. 
     Also, in case when the exposure light (ultraviolet light) from the secondary light sources has a very sharp directivity and interference of the exposure light in adjacent areas can be omitted, the light shielding device, such as the light shielding plate are not necessary. 
     The correction lens and light quantity correcting filter can be incorporated as a unit with focusing lens system and hole plate in the secondary light source. 
     Since the focusing lens system in the secondary light source is intended to widen the effective width of the exposure light, concave lenses or combination of multiple lenses can be used as the focusing lens system. 
     The hole plate in the secondary light source is not necessary for practical use in cases that the diameter (or the optical object point) of the exposure light emitted from the secondary light source is sufficiently small.